If I have a point to make... computerized alignment equipment is not likely to come loaded with 40 and 50 year old specs, much less a pallet of books. Those specs are largely irrelevant and just like your local parts store, the "database" is limited to late model stuff where the volume (money) is.

Here is where we're going to have a rare disagreement Doc. A pallet of books isn't necessary. Alignment is alignment, and in the technical age we live in, "ancient" is irrelevant. It's setting caster camber and toe, not timing a V-16 Duesenberg. Regardless there is close to unlimited information in pretty much everyone's pocket, and it only requires a minimal amount of effort to obtain. It took me less than 30 seconds to type "1975 Camaro alignment specs" into a search engine and pull up every last bit of information you would ever need about aligning a 1975 Camaro, along with diagrams, pictures and videos. It only takes someone with even a little dedication to their craft to make it happen.

The local parts store database isn't that limited either. I routinely pull up parts for early 50's cars and forward through standard parts house online catalogs. I do my own research because I know it's there. The kid behind the counter staring like a deer in headlights when you ask for an old part doesn't serve you because he doesn't want to make the effort, just like the "front end guy" in question here...

Maybe we are overthinking this. Now that I think about it some more, and after reading the rest of the comments, it may not be all that important how the sub-frame was installed. Within reason, that is. The main thing to consider is that if you can hit the Chevy specs with the current installation, it should be pretty dang close if not spot on.

The final configuration may not look like it did on the original car, but who cares as long as it drives straight and doesn't wear out the tires before it should.

The only real difference here is that the 56 had dive in the suspension and the Camaro along with newer vehicles has anti-dive.
This anti-dive will make the 56 handle way better than it's original suspension ever did.

Camber and toe should be the same as it was on the Camaro. Caster could be different, depending on if the subframe is in the same level to the ground as it was on the Camaro. You'll have to check the caster inclination on the spindle and adjust accordingly to the Camaro specs. A bit of extra caster will just help it to go straight at the expense of being a little harder to steer.

I took the '56 to a local guy who has been doing this for a number of years, and is more into old cars as a rule. He set it up with the Camaro specs, took his time, and got it dialed in. I haven't driven it on the road, as it doesn't have a rear window or trunk lid or hood right now, but I believe it'll be fine.

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Just saying...guess I'm just lucky. Shops wouldn't touch the 31. I did the set up myself. Wish my shop aligned vehicles drove as well as the 31...

Aligning a straight axle is easy to do at home (and pointless to take to a shop unless you're mechanically illiterate). Aligning an IFS car definitely needs some kind of equipment.

I will preface my next comments by saying my dad has been an automotive mechanic for the last 45+ years and has done alignments on all our and customer vehicles with clips, front suspension kits, frame swaps, etc.

So to answer the original question, you always use the specs of the suspension you're aligning. In this case, it's a 2 wheel alignment so nothing matters but what the front suspension was from. So you align it to 75 Camaro specs. From the suspension standpoint, it's no longer a 56 Ford so using the alignment specs for the Ford would be terribly incorrect and it would probably handle like a shopping cart. Yes, a good alignment shop can make it handle a little better by tweaking the numbers, but the baseline is going to be 75 Camaro. Some shops have specs for older stuff, and some don't. Really you just have to google, get the specs for a 75 Camaro, and give him that. Done.

I took the '56 to a local guy who has been doing this for a number of years, and is more into old cars as a rule. He set it up with the Camaro specs, took his time, and got it dialed in. I haven't driven it on the road, as it doesn't have a rear window or trunk lid or hood right now, but I believe it'll be fine.